1. Field of the Invention
The present invention relates to a variable gain amplifier circuit and a gain control method thereof. More particularly, the invention-relates to a variable gain amplifier circuit comprising variable gain amplifiers for low frequency concatenated in a multistage chain and a gain control method thereof, suitable for use in an integrated circuit (hereinafter abbreviated to IC) in which a variable gain low-noise amplifier for high frequency exists with the variable gain amplifiers for low frequency.
2. Description of the Related Art
FIG. 1 shows a typical circuit topology of conventional wireless receivers of low intermediate frequency (low IF) architecture. A high frequency signal received by an antenna 1 is amplified by a variable gain low-noise amplifier 2, mixed with a reference wave from a local oscillator circuit (OSC) 3 by a mixer 4, and thereby converted into a low frequency signal. This low frequency signal further passes through, normally, a plurality of variable gain amplifiers for low frequency 5, 7, and 9 and filters (F) 6 and 8 (this section consists of n stages of low-frequency signal amplifiers, ST1 through STn, one stage consisting of a couple of a variable gain amplifier for low frequency and a filter, in order to amplify the signal up to a required power level). Then, an AD converter (AD) 10 converts the thus amplified signal into a digital signal and a demodulator (DEM) 11 demodulates the signal into data of communication.
Also, it is widely practiced to substitute a fixed gain low-noise amplifier for the variable gain low-noise amplifier 2. However, because the domain of linear amplification of receivers becomes narrower due to recent IC design for good performance with a lower supply voltage, it is more advantageous to provide the amplifier with a variable gain function to restrain its output amplitude range, for example, to reduce the gain for input signals with a high power level. The variable gain amplifiers for low frequency 5, 7, and 9 adjust the signal level so that the level of the signal input to the AD converter 10 becomes a predetermined value. Circuits for controlling the variable gain low-noise amplifier 2 and the variable gain amplifiers 5, 7, and 9 are not shown in FIG. 1.
FIG. 2 shows a typical signal waveform of carrier signals which are actually used in wireless communication. Both the signal input to the variable gain low-noise amplifier 2 and the signal input to the variable gain amplifier 5 have the waveform shown in FIG. 2. However, the signal input to the former amplifier normally has a high frequency of a few GHz, whereas the signal input to the latter amplifier normally has a frequency of a few MHz because it is converted to a low frequency by the mixer 4.
As shown in FIG. 2, the waveform sequence includes a rise period T1 of the waveform and a preparation period T2 for data signal reception. Taking advantage of the rise period T1 of the waveform and the preparation period T2 for reception, gain control of the variable gain low-noise amplifier 2 and the variable gain amplifiers 5, 7, and 9 must be performed to adjust the gain of each above amplifier to an optimum gain which must be maintained during a data signal period T3.
However, the preparation period T2 for reception is generally defined to be very short; for example, this period is only 4 μs, as defined by the standard (IEEE 802.15.1) for short range data communication. Therefore, a method for controlling the gain of each above amplifier to be optimum for a very short period is needed.
As for prior art of such gain control, as is shown in FIG. 3, a receiver circuit which comprises a low-noise amplifier 31 which outputs a signal with variable gain adjusted by control signal C1, a mixer 32 which mixes its input RF (high frequency) signal with a local oscillation signal 33 to convert the signal into an intermediate frequency signal and outputs the intermediate frequency signal with variable gain adjusted by control signal C2, a variable gain amplifier 34 for low frequency which outputs a signal with variable gain adjusted by control signal C3, and a control circuit which generates the control signals C1, C2, and C3 which vary, depending on the output of the variable gain amplifier 34, and a method for controlling the receiving gain have been known (for example, refer to Japanese Patent Document Cited 1).
In this prior art example, the level of the signal output from the variable gain amplifier 34 for low frequency is detected and, based on the detected signal level, the control circuit 35 controls the gains of the low-noise amplifier 31, mixer 32, and variable gain amplifier 34 to adjust each gain to an optimum value.
Similar topology is found in a receiving circuit of a wireless LAN chip provided by Philips Semiconductors, Inc. (refer to Non-Patent Document Cited 1).
[Japanese Patent Document Cited 1]
    Japanese Published Unexamined Patent Application No. 2002-016462[Non-Patent Document Cited 1]    ISSCC2001/SESSION13/WIRELESS LAN/13.5, VISUALS SUPPLEMENT p.164